<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			* {
				margin: 0;
				padding: 0;
			}
			#myCan {
				background-color: skyblue;
			}
		</style>
	</head>
	<body>
		<canvas id="myCan" width="400px" height="400px"></canvas>
		<script type="text/javascript">
			var oC = document.getElementById('myCan');
			var ctx = oC.getContext('2d');
			// 弧度与角度的换算公式
			// 弧度 = 角度*Math.PI()/180
			// ctx.arc(200,200,80,0,2*Math.PI,true);
			ctx.arc(200,200,80,0,90*Math.PI/180,true);
			ctx.fill();
		</script>
	</body>
</html>
